Controlling write current overshoot characteristics in hard disk drive (HDD) write drivers is challenging since it depends on many variables. Some variables are dynamic in nature due to changing environment, such as variation in steady state write current and power supply voltage. Other system level variables, such as an output transmission line connecting to a head assembly and head load characteristics, may change significantly in different HDD products.
Varying HDD manufacturer requirements also add to the challenge. Different HDD manufacturers require different relationships between steady state and overshoot currents. Consequently, it is difficult to design a base core of circuitry to be used in multiple products for many customers since different HDD requirements often lead to designs that must be quite different.
Several previous attempts at controlling hard disk drive write currents used analog implementations and analog representations of dynamic variables. Such previous designs used assumptions on system level variables in order to calculate the appropriate write driver output voltage. One of the consequences of these assumptions concerns how the effects of non-ideal system variables, such as transmission line losses can be dealt with in a practical manner. Basing theoretical calculations on assumptions can result in a large margin of error, causing poor performance. Also, analog implementations require more power than complementary metal oxide semiconductor (CMOS) digital implementations and are complex and sensitive, adding time and risk to the process of designing an optimized analog implementation.